ESTRO 2024 - Abstract Book

S3632

Physics - Dose prediction, optimisation and applications of photon and electron planning

ESTRO 2024

2163

Digital Poster

Robust mixed electron photon radiation therapy: an alternative to bolus for soft tissue sarcoma

Veng Jean Heng 1 , Yee Man Tai 1 , Monica Serban 2,3,1 , Marc-André Renaud 4 , Carolyn Freeman 5 , Jan Seuntjens 2,3,1

1 McGill University, Medical Physics Unit, Montreal, Canada. 2 Princess Margaret Cancer Centre, Radiation Medicine Program, Toronto, Canada. 3 University of Toronto, Department of Radiation Oncology, Toronto, Canada. 4 Gray Oncology Solutions, ., Montreal, Canada. 5 McGill University Health Centre, Gerald Bronfman Department of Oncology, Montreal, Canada

Purpose/Objective:

Mixed electron-photon beam radiation therapy (MBRT) is an emerging technique which utilizes both external electron and photon beams. It exploits the steep dose falloff and high surface dose of electrons while maintaining target conformity by leveraging the sharp penumbra of photons. This study investigates the dosimetric benefits of MBRT when used for soft tissue sarcoma (STS) patients. In particular, we assessed the possibility of circumventing the use of tissue-equivalent bolus when treating with MBRT in superficial cases that had required them with photon-based standard of care. In past modulated electron studies, shortened SSD setups were used to sharpen electron penumbras. Although this had dosimetric benefits, it increased the risk for patient collision and required the translation of the treatment couch to accommodate gantry angle rotation. As photons with sharp penumbras are available with MBRT, we investigated the viability of a standard 100 cm SAD setup with MBRT. A retrospective cohort of 22 STS of the lower extremity treated with conventional photon-based VMAT were replanned with MBRT. Both VMAT and MBRT treatments were planned on the Varian TrueBeam linac using the Millenium multi-leaf collimator. No electron applicator, cutout or additional collimating devices were used for electron beams of MBRT plans. Bolus present in 10 plans was set to be air during MBRT planning. MBRT treatment planning is performed on Brems, an in-house web-based treatment planning system. MBRT plans consist of a combination of 6 MV photon and electrons from 5 energies (6 to 20 MeV). They are optimized robustly to setup errors by calculating beamlets in 6 additional position-shifted scenarios. For each such scenario, a shift of 5 mm is artificially introduced in a cartesian direction. The plans are normalized such that 95% CTV is covered by the prescription dose of 50 Gy, as averaged over all scenarios. This is in contrast to the VMAT plan which was non robustly optimized on the PTV as currently done in standard clinical practice. For a fair comparison, both plan types were recalculated with EGSnrc Monte Carlo simulations using an identical beam model. To assess the impact of a standard SAD vs. shortened SSD setup, 2 MBRT plans were generated for each patient: one with a standard SAD of 100 cm and one with a virtual SAD of 80 cm. The optimization constraints were kept identical for both plans. The standard SAD plan was finally used to compare with the standard of care VMAT plan. DVH metrics were averaged over all 22 patients and compared through a Wilcoxon signed-ranked test. Material/Methods:

To ensure that the plans were deliverable, 5 plans were selected to be delivered onto a MapCHECK detector with collapsed gantry angles. The dose to each diode detector in the MapCHECK device was calculated using EGSnrc.